The field of the invention relates generally to grounding devices and, more particularly, to grounding devices for electric machines having stray electrostatic charges on a rotatable shaft.
At least some known electric machines, such as brushless alternating current (BLAC) permanent magnet motors, are driven by pulse width modulation (PWM) technology and create a capacitance charge between the coils and the rotor. This capacitance charge results in substantially induced currents in the motor shaft.
Other known electric machines such as three-phase motors are driven by variable speed drives that induce a voltage or electrostatic charge on the motor shaft. Some variable speed drives utilize pulse width modulation technology to vary the speed of AC motors, which allows use of less expensive AC motors in applications where more expensive DC motors are used. A drawback of the use of AC motors with variable speed drives is that higher common mode voltage (CMV) is generated by the variable speed drive, which may increase induced currents on the shaft.
In some known motors, voltage or charges on the motor shaft induces current flow through the shaft bearings to the motor frame and then to ground. While the motor is running, the bearings become more resistive to current flow, causing a buildup of charge on the shaft surfaces. Over a short period of time, electrical charges can build up to a high level. As the electrical charges pass the threshold level of the least electrically resistant path, such as through the ball bearings on the shaft, a discharge of electrical energy passes through the least resistant path. This discharge causes electric discharge machining, which can affect the surfaces of the bearing races and the balls in the bearing if the least resistant path is through the bearings. This discharge may create fusion craters, and particulate matter from the crater formation may remain inside the sealed bearing. Both the fusion crater and the particulate material in the bearing act to disturb the free flow rotation of the bearing, which may reduce the operating lifetime of the bearing and/or the electric machine.
A number of mitigation technologies have been used to overcome this problem. The technologies include conductive bearing grease, insulating the bearings, and using various brushes made of metal or other materials to ground the shaft. Such brushes are typically mounted in rigid holders and may be expensive and require lengthy installation procedures. Moreover, the brushes are mounted to the exterior of the motor and therefore exposed to the environment surrounding the motor.